Geology of Artesian Basin in South Dakota. 383 



that the Dakota sandstone lies on quartzite or granite from Yank- 

 ton to Jamestown and not on Jura-Triassic rocks. 



Here are a few of the facts : at Vermillion quartzite was 

 reached at 630 feet below the surface; at Mitchell, 645 feet (with 

 light flow) ; at Plankinton, 760 feet (this lies west of James 

 river) ; at Tyndall, 735 feet; at Scotland, 548 feet; at Vilas, 462 

 feet; at Iroquois, 1,098 feet (147 feet above Hume) ; at Hume, 

 802 feet; Aberdeen, 955 feet. (Stopped at hard bottom.) This 

 last is either granite or quartzite, probably the former. 



This wide projection of these formations westward is along 

 the trend of the great Pre-Cambrian systems that stretch from 

 Lake Superior across Minnesota, and far into southeastern South 

 Dakota. Why is this idea not the key to the peculiar distribution 

 of the drift in the Dakotas? If the theory of a series of centers 

 •of glaciation is true why may it not be that the ice mass moved 

 from North Minnesota southwestward, rather than from the 

 Turtle Mountain country or Lake Winnipeg? That would put 

 the heavy moraine in the right place across the Dakotas, and ac- 

 count also for the Laurentian bowlders in the drift beyond the 

 Missouri. 



The next feature of the section of this basin is the Cretaceous, 

 w^hich system is represented by the Colorado group, consisting or 

 the Pierre, Niobrara and Benton formations. These forma- 

 tions in descending order overlie and overlap the Dakota sand- 

 stone. This last named is a widespread deposit ranging from 

 only a few feet to a hundred or more feet in thickness ; it is per- 

 sistent throughout the artesian region, and is the rock furnishing 

 the tremendous flow of the wells. 



A short study of this Cretaceous section will indicate the 

 artesian conditions that prevail in this region. 



If you start anywhere on the Pre-Cambrian border you will 

 find these formations overlying granite or quartzite. Sometimes 

 as a thin trace — a mere feather edge, as it were, of shale or chalk 

 — but usually appearing in force as one approaches the James 

 river valley. The only surface exposure of the Dakota sandstone 

 is found in the extreme southeastern portion of the state, where 

 it is seen to pass under the Benton shales. These shales and the 

 Niobrara are both found to overlap the Dakota to a greater or less 

 extent. The Benton shales are from 90 to 100 feet thick, ami 

 consist largely of ''shales and laminated clays with some layers 



